Despite the benefits of pharmacotherapy for early-onset bipolar disorder, bipolar youths report high rates of relapse and poor psychosocial functioning. However, studies of early-onset bipolar disorder have not identified environmental variables that influence the course of the disorder. Based on studies with adults that indicate an important relationship between stress and bipolar disorder, and findings that adolescents experiencing high levels of stress are at increased risk for depressive episodes, this study proposes an exploratory, prospective examination of stress as a predictor of symptomatic and psychosocial outcome among bipolar youths. A total of 40 adolescents with a bipolar I diagnosis will participate in assessments of stress, symptoms, and behavioral problems at 3-month intervals for 12 months. Youths reporting high levels of stress are expected to experience less improvement in bipolar symptoms and behavior problems over time, compared to adolescents reporting low levels of stress. Types of stressors will also be examined to identify key targets for psychosocial interventions. Chronic stressors are expected to have a more prominent role in predicting symptoms and behavioral problems than episodic stressful events. Family-related stressors are expected to have a stronger influence on symptom fluctuations over one year than school and peer-related stressors. Findings will suggest future directions for understanding the types of stressors that influence the cycling of early-onset bipolar disorder, and the causal mechanisms involved.